A variety of harmful heavy metal ions are contained in some industrial waste water and must be sufficiently removed by a waste water treatment in view of the prevention of environmental pollution. In addition, heavy metal components contained in river water and groundwater also adversely affect the human body and sufficient measures must be taken against these heavy metal components when the river water or ground water is used as, for example, drinking water or water for food processing. Furthermore, metal having possibilities of contaminating as, for example, hydrogenation catalysts during the production of, for example, edible oils and oils for food processing must be removed as much as possible, because they affect storage stability and human bodies.
Boron, a kind of metalloids, or a boron compound is widely distributed in the natural world and is an essential element to the human body. However, it has been verified that an excess intake of this element adversely affects the human body. Cases which are supposed to be caused by human-induced pollution with boron components in rivers or ground water have been reported, which invites apprehension of bad influence of recycling such water. In addition to boron, for example, arsenic and arsenic compounds are harmful to the human body and must be removed as much as possible from, for example, drinking water which is suspected to be contaminated by these substances.
If a liquid is used as, for example, irrigation water, water for food processing, edible oils or oils for food processing, insoluble impurities must be removed, in addition to such metallic components and metalloid components.
Under these circumstances, the present invention has proceed investigations to provide a filter which can efficiently capture and remove these metal ions or metalloid ions and compounds thereof from a liquid to be treated such as water or edible oils, or further, which can efficiently concurrently remove insoluble impurities contained in the liquid to be treated to thereby purify the liquid, and to develop a technique which can purify a liquid to be treated such as water and oils using the filter.
An ion exchange resins has been conventionally widely used to remove harmful metal ions and to capture useful metal ions contained in, for example, irrigation water or waste water, but its effect of selectively adsorbing and separating low concentration metal ions is not necessarily sufficient.
A chelate resin having a property of forming a chelate with metal ions to thereby selectively capture these ions has excellent selective capturing property to metal ions, especially to heavy metal ions, and has also been effectively employed to remove or capture heavy metals in the field of water treatment. However, most of these chelate resins are prepared simply by introducing a chelate-forming functional group such as iminodiacetic acid, and do not always exhibit a satisfactory chelate-forming capability.
Conventional ion exchange resins and chelate-forming resins are bead-like resins having a rigid three-dimensional crosslinked structure imparted by a cross-linking agent such as divinylbenzene, and a rate of diffusion and permeation of metal ions or regenerates into the resins is low, and therefore, a treatment efficiency is low. In addition, in the case of disposal of the used resins which are not recycled, problems such as difficulty in incinerating and complicated volume reduction of used resins have been greatly pointed out.
As a possible solution to these problems in bead-like chelate-forming resins, a fibrous or sheet-like chelate material has been proposed (Japanese Unexamined Patent Application Publication No. 7-10925). This fibrous or sheet-like chelate material has many advantages as follows. For example, this material has a large specific surface area, and includes a chelate-forming functional group, where metal ions are adsorbed or eliminated, on its surface and therefore has an improved adsorption and elimination efficiency. In addition, incineration and other treatments of the chelate material can be easily conducted. However, the production of the fibrous or sheet-like chelate material requires a complicated process with the use of ionizing radiation, and a number of disadvantages have been pointed out in, for example, facilities, safety, and production costs.
Furthermore, the conventional chelate resins or fibrous or sheet-like chelate materials are not expected to remove insoluble impurities which contaminate a liquid to be treated in not a little amount, although they exhibit a capability of capturing metal ions to some extent as mentioned above. Accordingly, to remove such insoluble impurities to thereby sufficiently purify the liquid to be treated, the insoluble impurities in the liquid to be treated must be removed prior to or subsequent to the removal of metal ions by, for example, the chelate resin, and a treatment comprising at least two steps must be employed to yield a reliable purification effect.
Under these circumstances, the present inventors have made investigations which are intended to develop a chelate-forming fiber which has a satisfactory capability of capturing metal ions, metalloid ions, or compounds thereof, is easy to be subjected to incineration, and can be produced by a simple and safe process at low costs. As part of the results of these investigations, the present inventors developed a chelate-forming fiber obtained by allowing a fiber having, in a molecule, a hydroxyl group, an amino group, or another reactive functional group, such as cotton or another vegetable fiber or silk or another animal fiber, to react with an amine compound (preferably, for example, D-glucamine, N-methyl-D-glucamine, or dihydroxypropylamine) represented by the following general formula (2) and having a capability of forming a chelate, or with an acid anhydride of a polycarboxylic acid (preferably, for example, nitrilotriacetic anhydride, ethylenediaminetetraacetic dianhydride, or diethylenetriaminepentaacetic dianhydride) represented by the following general formula (4) to thereby introduce a chelate-forming functional group represented by the following general formula (1) or a chelate-forming functional group represented by the following general formula (3). They previously filed a patent application on this technology.                 [wherein G represents a residue of a chain sugar alcohol or a residue of a polyhydric alcohol, and R represents        a hydrogen atom, a (lower) alkyl group, or —G (wherein G has the same meaning as defined above and may be a group identical to or different from the aforementioned G)]        [wherein G and R have the same meanings as defined above]        (wherein each of R1, R2, and R3 is a lower alkylene group, and n denotes an integer of 1 to 4)         (wherein R1, R2, R3, and n have 1 to the same meanings as defined above)        
The chelate forming fiber having the chelate-forming functional group represented by the general formula (1) or (3) introduced into a fiber molecule was first developed by the present inventors and the fiber itself was novel at the time when the present patent application was filed.
The chelate fiber having the introduced functional group represented by the general formula (1) has a satisfactory capability of capturing, as a chelate, metalloid ions or compounds thereof. The chelate fiber having the introduced functional group represented by the general formula (3) has a satisfactory capability of capturing, as a chelate, copper, zinc, nickel, cobalt, and other heavy metal ions and compounds thereof.
Such a chelate fiber having the chelate-forming functional group introduced into a fiber molecule has an excellent chelate-capturing capability and can be easily produced and is thus very useful in practice in comparison with conventional granular chelate resins.
Accordingly, it is a first object of the present invention, by making use of the activity of capturing metalloids or heavy metals, or compounds thereof, and by making use of the activity of removing insoluble impurities, to provide a chelate-forming filter which can capture or remove metalloids, heavy metals, or compounds thereof and can concurrently remove insoluble impurities from a liquid to be treated and to provide a process for the purification of a liquid using the filter. The activity of capturing metalloids, heavy metals, or compounds thereof utilizes the specific characteristics of the chelate-forming fiber, i.e., satisfactory chelate-forming property, and the activity of removing insoluble impurities utilizes the specific characteristics that the chelate-forming fiber is in the form of a fiber.
Yet another object of the present invention is to provide a process for easily producing a chelate-forming filter which has both an optimum chelate-forming capability for various ions to be captured, and the capability of removing insoluble impurities.